Fonds consists of correspondence and literary manuscripts, documenting all aspects of MacMillan’s career. Biographical background is provided by his own notes on his life and family history. From his student years come certificates and testimonials in support of his application for a Rhodes Scholarship (1904-1909). His war experience is recorded in letters from the front preserved by his family, and by a personal diary for 1917. There are also approximately twenty photographs of Macmillan and his family taken between about 1905 and 1940.

Macmillan's correspondence includes files of letters to his wife (1917-1936) on his war service and political affairs; personal and social letters (1923-1929); political correspondence, including several letters from McKenzie King (1926-1946); letters concerning McGill, including a number from Sir Arthur Currie and from Stephen Leacock (1920-1947); post-retirement correspondence on McGill and public affairs; and correspondence concerning his publications (1920-1960). There are also files of invitations to events at McGill and elsewhere.

The manuscripts fall into two categories: political speeches and literary manuscripts. Besides notes and texts for his own speeches, there are also texts of, and newspaper clippings about speeches written by Macmillan for Sir Arthur Currie and others (1920-1950). Drafts and typescripts of his literary works are supplemented by notes, clippings and correspondence.

Collection contains two financial documents related to McGill College. The first is a memorandum between the College and its creditors in which the College agrees to pay out dividends and pay in installments towards the balance of the College's debt. It is signed by the individual creditors as well as by the principal of McGill College, Edmund A. Meredith. The memorandum contains a list of creditors of McGill College with their names and occupations listed, including William Lyman, druggist, Robert Abraham, printer, John Keller, merchant, and many others. A note dated 24 March 1847 with a response from representatives of the creditors is pasted on. The second document is a ledger sheet dated from 1844 to 1847 with the caption: "The Directors of McGill College to Joseph Hitchens."